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The Oh-My-God particle was the highest-energy cosmic ray detected so far (as of 2019[update]), by the Fly's Eye detector in Dugway Proving Ground, Utah, US, on 15 October 1991. Its energy was estimated as (3.2±0.9)×1020 eV, or 51 J. This is 20 million times more energetic than the highest energy measured in electromagnetic radiation emitted by an extragalactic object and 1020 (100 quintillion) times the photon energy of visible light, equivalent to a 142-gram (5 oz) baseball travelling at about 26 m/s (94 km/h; 58 mph).
Assuming it was a proton, this particle traveled at 99.99999999999999999999951% of the speed of light, and its Lorentz factor was 3.2×1011. At this speed, if a photon were travelling with the particle, it would take over 215,000 years for the photon to gain a 1 cm lead as seen in Earth's reference frame.
The energy of this particle is some 40 million times that of the highest energy protons that have been produced in any terrestrial particle accelerator. However, only a small fraction of this energy would be available for an interaction with a proton or neutron on Earth, with most of the energy remaining in the form of kinetic energy of the products of the interaction. The effective energy available for such a collision is √, where E is the particle's energy and mc2 is the mass energy of the proton. For the Oh-My-God particle, this gives 3×1020 eV.
While the particle's energy was higher than anything achieved in terrestrial accelerators, it was still about 40 million times lower than the Planck energy. Particles of such energy would be required in order to explore the Planck scale. A proton with that much energy would travel 1.665×1015 times closer to the speed of light than the Oh-My-God particle. As viewed from Earth it would take about 3.579×1020 years, or 2.59×1010 times the current age of the universe, for a photon to gain a 1 cm lead over a Planck energy proton as observed in Earth's reference frame.
Since the first observation, at least 72 similar (energy > 5.7×1019 eV) events have been recorded, confirming the phenomenon. These ultra-high-energy cosmic ray particles are very rare; the energy of most cosmic ray particles is between 10 MeV and 10 GeV. More recent studies using the Telescope Array have suggested a source for the particles within a 20-degree radius "warm spot" in the direction of the constellation Ursa Major.
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- The blazar Markarian 501, measured in 1997.
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- "Physicists spot potential source of 'Oh-My-God' particles". sciencemag.org. 8 July 2014.